BIOLOGY OF RUSTS AND RE SI STANCE : MODERATOR'S SUMMARY 95 



are brought into closer genetic balance. The operation of genetic feedback 

 was first demonstrated through computer studies on imaginary populations. 

 These studies showed that, under appropriate conditions, a system in which 

 the relative numbers of those eating and those being eaten out of 

 balance will generate a series of regular fluctuations, the amplitude of 

 which will decrease as stability is approached. The decrease in amplitude 

 is mediated through genetic feedback. Pimentel has succeeded in demon- 

 strating the operation of genetic feedback in at least one parasitic system 

 (the house fly and a parasitic wasp which feeds on house-fly pupae) in 

 the laboratory. He also points to the evidence gained through use of the 

 myxomatosis virus to control the european rabbit in Australia. Anyone 

 who is familiar with the history of wheat stem-rust in North America will 

 also be familiar with the operation of genetic feedback. 



A second hint as to how natural systems may be self-regulating 

 originated with the suggestion by E . B. Ford (1965) that resistance to 

 disease may be one of the factors that can lead to continued maintenance 

 of two or more alleles within a single population, and hence to stable 

 genetic polymorphism. This possibility was examined (for the rusts) by 

 C. J. Mode (1958), and, more recently, by myself (Person, 1966). An 

 essential feature of the models we have proposed is that regulation is 

 achieved through more or less regular cyclic fluctuations in the frequencies 

 of "major" resistance alleles. KTiat is envisaged is a series of genetic 

 changes, mediated by genetic feed-back, which repeat as time progresses 

 so that the parasite is presented with a host population whose genetic 

 composition is constantly changing. Keeping in mind that it is through 

 continuous replacement of host varieties that stem rust of wheat has been 

 held in check, these models seem not to be entirely without merit. (As 

 well, the introduction and use, by Borlaug, of multiline varieties has 

 been successful; in this approach the parasite is presented with spatial, 

 rather than temporal, discontinuities of host genotype. It is possible 

 that both kinds of discontinuity co-exist in naturally occurring 

 populations .) 



Hints of this kind can be taken as only crudely suggestive of the 

 mechanisms for self- regulation that may actually exist in nature. For 

 this reason it will probably not be possible to select, in advance, any 

 "best" approach to the solution of the white pine blister rust problem. 

 It is my opinion that the populational-ecological approach must be given 

 prominance and, in such a case, that a great deal of research will be 

 needed. It is in this undertaking that I see the opportunity, mentioned 

 earlier, of discovering important new principles in the biology of 

 disease . 



LITERATURE CITED 



Ford, E. B. 1965. Genetic polymorphism. Faber and Faber London. 

 101 p. 



Mode, C. J. 1958. A mathematical model for the co-evolution of 

 obligate parasites and their hosts. Evolution 12: 158-165. 



Person, C. 0. 1966. Genetic polymorphism in parasitic systems. 

 Nature 212: 266-267. 



Pimentel, D. 1961. Animal population regulation by the genetic feed- 

 back mechanism. Amer. Natural. 95: 65-79. 



van der Plank, J. E. 1968. Disease resistance in plants. Academic 

 Press, New York and London. 206 p. 



Williams, K. 1963. Genetic principles and plant breeding. Blackwell 

 Scientific Publication, Oxford. 512 p. 



